1. Field of the Invention
The present invention relates to a multistage relay device provided between two relay channels in accordance with an asynchronous transfer mode (ATM), in which an ATM cell is transmitted from one to the other relay channel. In particular, the multistage relay device is connected to an exchange for exchanging signals in accordance with a synchronous transfer mode (STM), interconnecting communication between exchanges, so as to achieve ATM communication.
2. Description of the Prior Art
An STM communication network, such as a telephone network, is widely used all over the world. In such a telephone network, a communication channel is established between two terminals via a switch so that an audio signal is exchanged between the terminals via the communication channel. Such a switch can generally handle 64 bps STM signals, or the like, but not other STM signals or ATM signals.
In the above telephone network, a switch is generally provided for every region. Thus, in order to establish communication from one region to another, a signal must be exchanged via two switches. Such communication via switches is achieved utilizing a multistage relay device provided to the respective switches. That is, a switch on the sender side delivers a signal to its associated multistage relay device, which then transmits the signal via a relay channel to the multistage relay device of a switch on the receiver side. The multistage relay device on the receiver side supplies the signal to the receiver switch.
In the above signal transmission between multistage relay devices, in which all of the regions allotted to the respective multistage relay devices are covered for communication, a large number of signals must be transmitted. Thus, efficient signal transmission (for instance, utilizing a multiplexing process, or the like) via a relay channel is desired, and adoption of ATM communication between multistage relay devices is therefore under consideration. However, it is preferable that it remains possible to utilize an extant switch which is capable of various convenient functions together with ATM devices.
In order to meet these requirements, coexistence of a multistage relay device for ATM communication and a switch for STM communication may provide a solution. For such coexistence, however, an intermediate multistage relay device for relaying a signal between two terminal multistage relay devices must tentatively expand a signal received via a relay channel to be a 64 kbps audio signal, and this expansion may result in the degrading of an audio signal.